Method of manufacturing display screens for cathode-ray tubes

ABSTRACT

After the development of the image of hardened pva obtained by exposure on the face plate panel of a cathode-ray tube and before the screen can be completely dried, the image is partly destroyed owing to the film of developing liquid which is left on the screen. This results in a poorly defined image and an upset colour balance. A sharply defined image is obtained by treating the screen with acetone immediately after, or even during, development.

United States Patent 1 Duinker et al.

METHOD OF MANUFACTURING DISPLAY SCREENS FOR CATHODE-RAY TUBES Inventors: Hand Dignus Duinker, Emmasingel,

Eindhoven, Netherlands; Eric Raymond Leng, Accrington, England US. Phillips Corporation, New York, N.Y.

Filed: June 10, 1971 Appl. No.: 151,941

Assignee:

US. Cl. 29/25.l8, 29/25.] 1, ll7/33.5 C Int. Cl. H0lj 9/00 Field of Search 29/25.], 25.l l, 25.17,

29/25.]8; ll7/33.5 E, 33.5 C, 33.5 CD

References Cited UNITED STATES PATENTS 10/1956 Epstein 29 2517 x June 19, 1973 6/1958 Watson ll7/33.5 CP 6/1971 Zeliotis 29/25.l7

Primary ExaminerRichard Bernard Lazarus Att0meyFrank R. Trifari 5 Claims, 10 Drawing Figures METHOD OF MANUFACTURING DISPLAY SCREENS FOR CATHODE-RAY TUBES The invention relates to a method of manufacturing display screens for cathoderay tubes, in which method the face plate panel is coated with a photosensitive layer which contains polyvinyl alcohol and a sensitizer and subsequently the photosensitive layer is locally hardened by exposure through a mask and the image is developed with an aqueous liquid, after which the face plate panel is coated with a non-reflecting layer, the hardened polyvinlyl alcohol is removed by etching, and the resulting uncovered portions of the face plate panel are coated with an electron-excitable luminescent material.

It is experienced as annoying that cathode-ray tube display screens reflect incident light. Hence, it has been proposed to coat those parts of the face plate panel which are not coated by an electron-excitable luminescent material with a non-reflecting material. (Dutch Patent application No. 6,804,370).

Usually there are applied to the face plate panel pigment dots having a diameter which greatly exceeds the diameter of any of those parts of the electron beams by which they are to be struck which are transmitted by the mask. Hence, the probability of a dot being struck is high.

In order to enable a maximum portion of the face plate panel to be coated with a non-reflecting layer, the pigment dots are to be as small as possible. In order to create a high striking possibility, the transmitted parts of the electron beams must be given diameters greater than those of the pigment dots.

Since the mask used to provide the pigment dot pattern on the face plate panel by exposure of a photosensitive layer is the same as that which is built into the cathode-ray tube, provisions must be made to ensure that the light beam which in the manufacture of the screen is transmitted by the mask produces a smalldiameter pigment, dot, whereas the parts of the electron beams which are transmitted by the mask in the finished tube have large diameters.

This is obtainable by initially using a mask having small apertures and, after completion of the dot pattern, increasing the size of the apertures in the mask by etching.

Another solution has been found, however, in which a pattern of small-diameter dots is formed by exposure through a mask having large-diameter apertures. This may be achieved by a method of exposure as described in Journal of the Society of Motion Picture and Television Engineers 65, 407-410 (1956).

In this method, the result of the exposure is a pattern of dots which each consist of a central portion of hardened polyvinyl alcohol (pva) surrounded by a region of partially hardened pva in a matrix of nonhardened pva. In the subsequent treatment with an aqueous liquid, the central portions and the regions immediately surrounding them are not removed, but the matrix is removed by washing. The dots are then dried.

It has been found that in this method the film of de-- veloping liquid which, after the treatment, was left on the face plate panel has post-developed the regions of partially hardened pva before, and during the beginning of, the drying process. However, the resolubilized pva has not been removed with the developer, as was the case during the treatment, but has resolidified on the screen during drying. As a result, the dots have fringes. Hence the dots are not circular any more and they have poorly defined diameters. Consequently, at a later stage of manufacture pigment dots of the same irregular shapes and deviating diameters will be produced, and also when the pva is removed by etching holes may be produced in the non-reflecting layer at other points than where pigment is to be deposited.

In the system using small pigment dotsto bestruck by parts of an electron beam having diameters greater than those of the pigment dots, the occurrence of pigment dots of irregular shapes and deviating diameters must be avoided at all costs, for in this system the size of a dot determines the intensity of the colour at the respective point. Dots of deviating shapes may give rise to an image which at one point is too green and at another point is too red or too blue.

[t has proved impossible to remove the aqueous layer left on the screen after development by heating or with the aid of a gas-stream at a rate such as to prevent postdevelopment.

Surprisingly it has now been found that a well defined image is obtainable when after development of the image the screen is treated with acetone.

This is the more remarkable as the use of other ketones, such as methyl-ethyl ketone and diethyl ketone, and of alcohols, such as methanol and ethanol, did not yield any improvement. A mixture: of acetone containing 20 percent or more of water also proved useless.

Accordingly, the invention relates to a method of manufacturing display screens for cathode-ray tubes, in which the face plate panel is coated with a photosensitive layer containing polyvinyl alcohol and a sensitizer, after which the photosensitive layer is locally hardened by exposure through a mask, the image is developed by means of an aqueous liquid and subsequently the face plate panel is coated with a non-reflecting layer, the hardened polyvinyl alcohol is removed by etching and the uncovered parts of the face plate panel are coated with an electron-excitable luminescent material, which method is characterized in that the developed image is fixed by a treatment with acetone.

The acetone treatment must be carried out immediately after development. Preferably, however, the treatment is begun with already during the latter part of the development.

Preferably the screen is sprayed with acetone, good results being achieved especially when the entire screen is simultaneously treated. Alternatively, however, the screen may be immersed in an acetone bath.

A treatment lasting only a few seconds is sufficient. To prevent any damage being done, the spraying pres sure should not be too high, but on the other hand a sufficient supply of acetone must be ensured. As a rule, a pressure ofa few tenths of an atmosphere will be sufficient.

The polyvinyl alcohol used may be one of the commercially available products having a degree of saponification of about 88 percent. The mean molecular weight preferably is about 100,000.

Suitable sensitizers are compounds of hexavalent chromium, such as alkali dichromates, ammonium dichromates, tetra-alkyl ammonium dichromate, trialkyl aralkyl ammonium dichromates and the like, and also compounds of pentavalent vanadium, such as ammonium metavanadate or tetra-alkyl ammonium metavanadate.

Furthermore other usual substances may be added to the photosensitive composition used, such as dipolar aprotic substances, for example dimethyl sulfoxide, N-methyl-a-pyrnolidone, 'y-butyrolactone, ethylene glycol carbonate, sulfolan, dimethyl acetamide, dimethyl formamide, hexamethyl phosphoramide and tetramethyl urea, and sparely volatile bases, such as tetra-alkyl-ammonium hydroxide.

The latent image obtained after exposure may be developed with water. Alternatively, however, a solution of the sensitizer may be used and after the image has been developed, fixed and dried, the screen may be exposed another time in order to complete the hardening of the partly hardened pva regions.

The screens provided with sharply defined pva dots according to the invention may be worked up into display screen by conventional techniques.

The method according to the invention is illustrated in outline by means of the FIGS. 1 to 10, which represent a cross sectional view through a glass plate 1.

In FIG. 1 the glass plate is coated with a photosensitive layer containing polyvinyl alcohol and a sensitizer.

In FIG. 2 the photo-sensitive layer is locally hardened by exposure through a mask, developed and fixed by a treatment with acetone.

In FIG. 3 a non-reflecting layer 3 is applied.

In FIG. 4 the hardened polyvinyl alcohol is removed by etching.

In FIG. 5 a photo-sensitive layer containing a first electron excitable luminescent material (e.g. red luminescent material) is represented by 4.

In FIG. 6 the photosensitive layer 4 has been locally hardened by exposure through a mask and developed.

In FIG. 7 a photo-sensitive layer containing a second electron excitable luminescent material (e.g. a blue luminescent material) is represented by 5.

In FIG. 8 the photo-sensitive layer 5 has been locally hardened by exposure through a mask and developed.

In FIG. 9, 6 represents a photo-sensitive layer containing a third electron excitable luminescent material (green luminescent).

In FIG. 10 the photo-sensitive layer 6 has been 10- cally hardened by exposure through a mask and developed.

EXAMPLES la. A glass face plate panel 1 (FIG. 1) for a display screen was coated with a layer 2 of an aqueous solution of polyvinyl alcohol (3.5 percent by weight) and ammonium dichromate (0.115 percent by weight). The layer was dried byheating with infrared lamps and by the passage of a stream of dry air. The layer was then exposed through a mask. The latent image was developed by spraying the screen for 1.5 minutes with water (spraying pressure 0.3 atmosphere). Starting during the last second of the development the screen was sprayed with acetone for 5 seconds (spraying pressure 0.2 atmosphere). The screen has then dried by heating with infrared lamps and by passing over a dry air stream, the pva dots 2 hardening thermally.

1b. The screen is then (FIG. 3) coated with a suspension of graphite 3 consisting of a 1,125 g of graphite, about 4,500 mi of deionized water, 30 ml of ethylene glycol, 2 ml of wetting agent, for example Dispergent L77 from Union Carbide, and 0.25 ml of a dispergent, for example Acrysol G 65 percent, a 65 percent by weight suspension of acrylate resin in water. The layer was dried by means of infrared lamps. Then the pva dots 2 were removed by etching with a 3 percent by weight hydrogen peroxide solution. (In an alternative embodiment of the method an 1 percent by weight potassium permangante solution was used instead of the hydrogen peroxide solution.) The screen was then washed with water.

After drying a screen was obtained which was coated with a graphite layer 3 (FIG. 4) in which a pattern of sharply defined circular holes has been formed. The screen was then worked up into a finished display screen (FIG. 10) by treating it with a photosensitive lacquer containing an electron-excitable luminescent material 4 (FIGS. 5 and 6), drying, exposing and developing, and by repeating these treatments using electron-excitable materials 5 (FIGS. 7 and 8) and 6 (FIGS. 9 and 10) luminescing in other colours.

2. The method of Example la was repeated, with the difference that as a developer an ammonium dichromate solution (0.003 percent by weight) was used instead of water. The developing time was 2 minutes and the pressure at which the developing liquid was sprayed on the screen was 0.1 atmosphere. Immediately after development of the image the screen was sprayed with acetone for 2 seconds (spraying pressure 0.3 atmosphere). The screen was then exposed again, but without the interposition of the mask. Subsequently the screen was sprayed with water (0.2 atmosphere) for 30 seconds in order to remove ammonium dichromate crystals, and then dried by spinning.

The screen was worked up into a finished display screen in the manner described in Example lb.

What is claimed is:

1. A method of manufacturing display screens for cathode-ray tubes, in which the face plate panel is coated with a photosensitive layer containing polyvinyl alcohol and a sensitizer, after which the photosensitive layer is locally hardened by exposure through a mask, the image is developed with an aqueous liquid, subsequently the face plate is covered with a non-reflecting layer, the hardened polyvinyl alcohol is removed by etching and the resulting uncovered portions of the window are coated with an electron-excitable luminescent material, characterized in that the developed image is fixed by a treatment with acetone. I

2. A method as claimed in claim 1, characterized in that the acetone treatment is begun during the latter part of the development.

3. A method as claimed in claim 1, characterized in that the screen is sprayed with acetone.

4. A method as claimed in claim 1, characterized in that a compund of hexavalent chromium is used as the sensitizer.

5. A method as claimed in claim 1, characterized in that the image is developed with an aqueous solution of the sensitizer, the screen being re-exposed after the acetone treatment. 

2. A method as claimed in claim 1, characterized in that the acetone treatment is begun during the latter part of the development.
 3. A method as claimed in claim 1, characterized in that the screen is sprayed with acetone.
 4. A method as claimed in claim 1, characterized in that a compund of hexavalent chromium is used as the sensitizer.
 5. A method as claimed in claim 1, characterized in that the image is developed with an aqueous solution of the sensitizer, the screen being re-exposed after the acetone treatment. 